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A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide - using angle-resolved photoemission electron spectroscopy - the band structure of the paramagnetic insulating phase of Ca2 RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the d xy orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca2RuO4.
Sutter, D., Fatuzzo, C. G., Moser, S., Kim, M., Fittipaldi, R., Vecchione, A., … Chang, J. (2017). Hallmarks of Hunds coupling in the Mott insulator Ca 2 RuO 4. Nature Communications, 8. https://doi.org/10.1038/ncomms15176